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Luminosity Profiles of Prominent Stellar Halos

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Luminosity Profiles of Prominent Stellar Halos Journal of the Korean Astronomical Society http://dx.doi.org/10.5303/JKAS.2018.51.4.73 51: 73 ∼ 88, 2018 June pISSN: 1225-4614 · eISSN: 2288-890X c 2018. The Korean Astronomical Society. All rights reserved. http://jkas.kas.org LUMINOSITY PROFILES OF PROMINENT STELLAR HALOS Hong Bae Ann and Hyeong Wook Park Pusan National University, 2 Busandaehak-ro, Geumjeong-gu, Busan 46241, Korea; [email protected] Received May 9, 2018; accepted June 30, 2018 Abstract: We present a sample of 54 disk galaxies which have well developed extraplanar structures. We selected them using visual inspections from the color images of the Sloan Digital Sky Survey. Since the sizes of the extraplanar structures are comparable to the disks, they are considered as prominent stellar halos rather than large bulges. A single S´ersic profile fitted to the surface brightness along the minor-axis of the disk shows a luminosity excess in the central regions for the majority of sample galaxies. This central excess is considered to be caused by the central bulge component. The mean S´ersic index of the single component model is 1.1 ± 0.9. A double S´ersic profile model that employs n = 1 for the inner region, and varying n for the outer region, provides a better fit than the single S´ersic profile model. For a small fraction of galaxies, a S´ersic profile fitted with n = 4 for the inner region gives similar results. There is a weak tendency of increasing n with increasing luminosity and central velocity dispersion, but there is no dependence on the local background density. Key words: galaxies: general — galaxies: photometry — galaxies: structure 1. INTRODUCTION difficult to observe. In the case of the Milky way, its halo luminosity is ∼ 1 per cent of the total luminosity, Stellar halos are thought to be ubiquitous in disk galax- and other spirals usually have halo luminosities similar ies (Sackett et al. 1994; Morrison et al. 1997; Lequeux to the Milky Way (Courteau et al. 2011). Since the et al. 1998; Abe et al. 1999; Zibetti et al. 2004; Zibetti surface brightness level needed to analyze the structure & Ferguson 2004; Zibetti et al. 2004; McConnachie et −2 of stellar halos is so faint (∼ 30 mag arcsec ), it re- al. 2006; Chapman et al. 2006; Kalirai et al. 2006; Seth quires highly precise observations and data reductions et al. 2007; de Jong et al. 2007; Mouhcine et al. 2007; are required to prevent observational artifacts. The first Helmi 2008; Jablonka et al. 2010; Ibata et al. 2014). detection of the stellar halo around disk galaxies was In the framework of the Λ cold dark matter (ΛCDM) reported for NGC5907 by Sackett et al. (1994), and cosmology (Springel et al. 2006), the existence of stel- confirmed by Lequeux et al. (1996) and Lequeux et al. lar halos around disk galaxies is very natural because (1998). Zibetti et al. (2004) applied an image stacking they are byproducts of hierarchical galaxy formation. technique to disk galaxies to derive the structural pa- They are thought to be formed by the accretion and rameters of their stellar halos. They showed that stellar disruption of dwarf satellites (Bullock & Johnston 2005; halos are moderately flattened spheroids with spatial Abadi et al. 2006; Font et al. 2006; de Lucia & Helmi luminosity distributions that are well described by a 2008; Font et al. 2008; Johnston et al. 2008; Gilbert et −3 power law (∼ r ). al. 2009; Cooper et al. 2010; Zolotov et al. 2010). There is mounting evidence that the accretion and disruption On the other hand, there are some disk galaxies of infalling satellites play a major role in building the with prominent stellar halos which are bright enough to stellar halos around disk galaxies (Searl & Zinn 1978; be visually identified. A well known example is NGC Johnston et al. 1996; Helmi & White 1999; Ferguson et 4594 (M104), a spiral galaxy, known as the Sombrero al. 2002; Bullock & Johnston 2005; Ibata et al. 2007; galaxy, with a Hubbe type of Sa (de Vaucouleurs et al. Johnston et al. 2008; Romanowsky et al. 2016). On 1991) however, it is sometimes considered as a peculiar the other hand, gas rich mergers in the early phase of elliptical galaxy (McQuinn et al. 2016) because of its the galaxy assembly (Brook et al. 2004), and in − situ bright stellar halo. The nucleus of NGC 4594 is fairly star formation (Font et al. 2011) were also proposed as bright, and there is a supermassive black hole in the cen- explanations of the origin of stellar halos. The ejec- ter (Kormendy 1988, 1996; Ho et al. 1997). More often, tion of in − situ stars formed at high redshift (z ∼> 3) the prominent stellar halo of NGC 4594 is considered as by subsequent major (Zolotov et al. 2009) and minor a massive bulge which contains ∼ 90% of the total lumi- mergers (Purcell et al. 2010) also could contribute to nosity (Kent 1988). However, the V −band minor axis stellar halos. profile of NGC 4594 (Jarvis & Freeman 1985) shows a ∼ The surface brightness of the stellar halos of spi- break at 55 arcsec which suggests two components, a ral galaxies is typically 10 magnitudes below the night stellar halo and central bulge. Detailed surface photom- sky background (Bakos & Trujillo 2012), making them etry of NGC 4594 (Burkhead 1986) also showed that the outer spheroidal component is a prominent stellar halo Corresponding author: H. B. Ann that can be well approximated by an ellipsoid with an 73 74 Ann & Park of prominent stellar halos and establish basic statistics. In particular, we aim to derive the S´ersic index of the luminosity profile of the prominent stellar halos to char- acterize the shape of the luminosity distribution. The S´ersic index n is known to be correlated with the lu- minosity, effective radius, and the central velocity dis- persion of galaxies (Graham et al. 2001; Mollenhoff & Heidt 2001; Graham 2002) when applied to ellipticals and the bulges of disk galaxies, Since prominent stel- lar halos are rare phenomena, we are also interested in their environment. This paper is organized as follows. The data and sample selection are described in Section 2, and the results of the present study are given in Section 3. Dis- cussion and conclusions are provided in Section 4. 2. DATA AND SAMPLE SELECTION 2.1. Selection of Prominent Stellar Halos We selected prominent stellar halos using the color im- ages provided by the Sloan Digital Sky Survey (SDSS) Data Release 7 (Abazajian et al. 2009). We used the Korea Institute of Advanced Study Value-Added Figure 1. Color-magnitude diagram of the sample galaxies Galaxy Catalog (KIAS-VAGC; Choi et al. 2010) to se- that show prominent stellar halos. lect the parent sample of target galaxies that have red- shifts less than z = 0.032, and r-magnitudes brighter axis ratio of b/a = 0.64. A clear distinction between than 17.77. The number of parent sample galaxies is the central bulge and the stellar halo can be seen in 44,244. We first selected 7,810 edge-on galaxies through the Spitzer 3.6-µ image (Gadotti & S´anchez-Janssem visual inspections of the SDSS color images in the par- 2012), in which the stellar halo is rounder than the cen- ent sample. Because of the large axis ratios (b/a) of the tral bulge, and dominates over the disk and bulge at prominent stellar halos, they are likely to be omitted large radius (r ∼> 215 arcsec). in the sample of edge-on galaxies selected by axis ratio Since most stellar halos are too faint to be observed criteria. Thus, it is necessary to conduct a visual inspec- by conventional methods, the number of galaxies with tion of the color images to see whether or not galaxies detailed observations of their stellar halos is quite lim- are edge-on. The selection criterion we have applied is ited. Therefore, there are no statistical samples of the the axis ratio of the disks and not the entire galaxy. structure and physical properties of stellar halos. On Sometimes, a well distinguished dustlane is considered the other hand, stellar halos have been well predicted as evidence of edge-on disks. by numerical simulations (Katz & Gunn 1991; Stein- Since we have selected the prominent stellar halos metz & Muller 1995; Brook et al. 2004) in the CDM by visual inspections of the color images without quan- scenario (White & Frenk 1991; Kauffmann et al. 1993). titative selection criteria, the selected sample is subject In a sense, these simulations failed to reproduce Milky to some personal bias, but we do not think we omit a Way type stellar halos whose mass fraction is less than significant fraction of prominent stellar halos. The final ∼ 1 per cent of the total luminous matter. Most stel- sample contains 54 galaxies, and corresponds to 0.1% lar halos that they modeled were too massive compared of the parent sample of 44,244 galaxies, and 0.7% of to observed stellar halos. However, Brook et al. (2004) the 7810 edge-on galaxies. This means that prominent was able to make different kinds of stellar halos depend- stellar halos are very rare. The basic properties of the ing on the supernova (SN) feedback mechanisms used. 54 galaxies are presented in Table 1, most of which are One model is similar to the faint stellar halos, often obtained from the KIAS-VAGC. observed, and another is similar to the massive stellar Figure 1 shows the color-magnitude diagram of halos that were reported in the previous studies (Katz galaxies in the final sample.
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